This invention relates to electric motors. One application is to small DC electric motors suitable for battery powered electric dry shavers, for example as disclosed in our co-pending U.S. patent application Ser. No. 886,482, a continuation of U.S. patent application Ser. No. 681,769, filed Apr. 29, 1976 and U.S. Pat. Nos. 4,092,581, and 4,115,920.
A known type of small DC electric motor suitable for this purpose comprises a tubular support, a shaft journalled for rotation within said support and extending therefrom to carry a rotor and a commutator at a free end thereof. The tubular support supports a central cylindrical permanent magnet. The rotor is bell-shaped, consisting essentially of a winding without a core but held in shape by adhesive or similar minimal support. The rotor surrounds the cylindrical magnet, and a magnetic shell surrounds the rotor to complete the magnetic circuit.
One disadvantage is that such bell-shaped rotors are difficult and thus expensive to manufacture in quantity. This is particularly true for battery powered motors where the supply voltage is low, e.g. 1.2 or 2.4 volts, thus requiring larger diameter winding leads. The central permanent magnet, requiring magnetization across a diameter, is also expensive. Furthermore a bell-shaped rotor has a very low moment of inertia. That can be a disadvantage in a number of applications, for example dry shavers, where the load is variable and a fly-wheel effect would thus be desirable.
Wound rotors are known per se and are less expensive than bell-shaped rotors, but a number of difficulties arise in adapting such DC electric motors to employ wound rotors in such applications as electric dry shavers wherein strict dimensional space limitations are present and wherein features of the driven appliance and of the power supply dictate a number of constructional demands on the motor itself.